In recent years, an MRAM (Magneto-resistive Random Access Memory) is highlighted as one of the memories which are expected to have superior characteristics as compared to conventional memories such as a DRAM and the like. A basic mechanism of an MRAM memory device is shown in FIGS. 12A and 12B. The MRAM memory device includes an insulation film 91 and ferromagnetic layers 92a and 92b installed on both sides of the insulation film 91. The magnetization direction of the ferromagnetic layer 92a is variable and the magnetization direction of the ferromagnetic layer 92b is fixed. When the magnetization directions of the ferromagnetic layers 92a and 92b are opposite to each other as shown in FIG. 12A, a resistance value of the insulation film 91 is increased. Thus, it becomes harder for current to flow from the ferromagnetic layer 92a to the ferromagnetic layer 92b. The MRAM uses this state as a “1” state of a memory device. Conversely, if the magnetization directions of the ferromagnetic layers 92a and 92b are identical with each other as shown in FIG. 12B, the resistance value of the insulation film 91 is decreased. Thus, it becomes easier for current to flow from the ferromagnetic layer 92a to the ferromagnetic layer 92b. The MRAM uses this state as a “0” state of a memory device.
A metal oxide film is used as the insulation film. If the resistance value of the metal oxide film is changed, the characteristic of the metal oxide film as the MRAM is also changed. Thus, the magnitude of a current required in writing “0” or “1” is changed. For that reason, a tight tolerance range is set with respect to the resistance value of the metal oxide film of the MRAM.
As a film-forming process of the metal oxide film, Japanese Patent Application Publication No. H6-172989 discloses a method (reactive sputtering method) for sputtering a target composed of a metal oxide. However, this method suffers from a problem in that the surface of the metal target is gradually oxidized during the sputtering. Japanese Patent Application Publication No. 2008-13829 discloses a method in which a metal oxide film is formed on a substrate by sputtering a metal-made target while introducing an oxygen gas into a sputtering apparatus. However, this method suffers from a problem in that the surface of the target is oxidized by residual oxygen and the resistance value of the metal oxide film becomes unstable. Japanese Patent Application Publication No. H6-49633 discloses a method in which a metal film is formed within one processing chamber and is then oxidized in another oxygen-supplied chamber while supplying oxygen. However, the use of two or more chambers leads to a reduction in overall apparatus throughput. Moreover, the oxygen introduced into the oxygen-supplied chamber remains within the chamber. Thus, there is a concern that residual oxygen might cause the resistance value of the metal oxide film to vary from substrate to substrate.
The metal oxide films formed by the methods disclosed in the three Japanese publications cited above are insufficient in terms of the stability of the resistance value among substrates. This poses a problem in that it is impossible to obtain a high yield rate.
Japanese Patent Application Publication No. 2009-65181 discloses an MRAM manufacturing method in which a processing chamber provided with two targets is used to perform gettering prior to the formation of a metal oxide film. However, this method does not provide motivation for the present disclosure, because the material of the metal oxide film (MgO in that disclosure) is directly used as a target.